In the $3 billion worldwide flowmeter market, the calorimetric flow sensor is fast becoming an industry standard. Significant advances over the past decade have enabled the calorimetric sensor to assume a very active and respected role in flow switching and flow metering of both liquids and gases.

The most common use of calorimetric sensors is in “flow-no-flow” applications where there is a need to detect the movement of air or liquids. Popular for its solid-state construction with no moving parts, the calorimetric flow sensor is impervious to the type of particulate matter that often spells doom for mechanical devices.

Historically, calorimetric switches have been significantly more costly than mechanical switches. But with new technological breakthroughs in both electronic and mechanical construction, calorimetric switching is growing much more cost efficient.

Calorimetric sensing technology allows measurement of flows that are below the inertia threshold of turbine or paddlewheel sensors. As such, they are very popular for chemical dosing, leak detection, and other extremely low-flow applications. On the opposite side of the spectrum, extended-range calorimetric sensors are now available, enabling the measurement of much higher flow rates (up to 60 feet per second for some liquids).

Traditional calorimetric sensors use two PTC thermistors, one of which is heated a predetermined amount above the other, which monitors the medium temperature. Flow of the medium conducts heat away from the sensor probe and the corresponding change in the heated thermistor’s resistance value is translated into an output (switching or analog, depending on sensor type) proportional to the rate of flow.

Significant advances made by some calorimetric manufacturers have resulted in accurate and extremely repeatable flow measurement performance. It has also been possible to incorporate the temperature sensing task (inherent to the calorimetric sensor) to provide a flow-and-temperature switch and a flow-and-temperature metering unit, thus providing the customer two sensors in one at an extremely attractive price.

Calorimetric sensors do not require the medium to be electrically conductive (as would a magnetic flowmeter, for example), which makes them a good fit for polymers, oil, grease, and numerous other nonconductive mediums. The technology is not reliant upon suspended particles for measurement accuracy (as would be the case with ultrasonic Doppler flowmeters). Further, the use of specialty metals (i.e., Hastelloy, Monel, Titanium, etc.) for calorimetric construction allows the sensors to support aggressive mediums, such as chemicals and acids.

Also, calorimetric sensors have minimal power requirements, enabling the use of battery-operated flow sensors for remote locations. When paired with wireless transmission devices, this enables Internet-based monitoring or dialup fault alarming for such applications as loss of flow to pumps, leaking pipes, and loss of lubrication oil flow or coolant flow.

Finally, emerging forms of microflow technology are enabling calorimetric sensors to measure liquid flows in ranges down to milliliters per hour, a valuable solution for many low-dosing applications.

About the Author
Larry Flanagan has over 25 years of experience in instruments and controls in engineering, sales, and product management. During his career, Mr. Flanagan has worked for a number notable industry names, including LFE Corporation, Crompton Instruments, and BTR. He is now the general manager of weber Sensors Inc. Mr. Flanagan can be reached at Larry.Flanagan@captor.com or 770 592-6630.

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